反式脂肪，又称为反式脂肪酸、逆态脂肪酸，属于不饱和脂肪酸（单元不饱和或多元不饱和）。

动物的肉品或乳制品中的脂肪酸多为饱和脂肪酸和顺式脂肪酸，虽有反式脂肪酸但含量低；人类食用的反式脂肪主要来自经过氢化进程所生产的部份氢化植物油。「植物油氢化」是在20世纪初期发明的食品工业技术，并于1911年被食用油品牌「Crisco」首次使用。氢化过程会改变脂肪酸的分子结构（让油更耐高温、不易变质，并且增加保存期限），如果将脂肪酸完全氢化并无反式与顺式脂肪酸差异， 但不完全氢化时，部份的脂肪酸结构会变成反式结构，产生反式脂肪。由于能增添食品酥脆口感、易于长期保存等优点，此类部分氢化植物油被大量运用于市售包装食品、餐厅的煎炸食品中。动物油在精制去味过程时，会生成少量的反式脂肪，反复煎炸也会产生反式脂肪。

Trans fats, or trans-unsaturated fatty acids, trans fatty acids, are a type of unsaturated fats that are uncommon in nature but became commonly produced industrially from vegetable fats for use in margarine, snack food, packaged baked goods and frying fast food starting in the 1950s. Trans fat has been shown to consistently be associated, in an intake-dependent way, with increased risk of coronary heart disease, a leading cause of death in Western nations.